1. Field of the Invention
The present invention relates to ion beam sputtering and, more particularly, to a method for monitoring ion beams with an electrically isolated aperture. Still more particularly, the present invention is directed to a method of fabricating optical mirrors.
2. Discussion of the Prior Art
U.S. Pat. No. 4,992,742 to Okuda et al. discloses a device for measuring the distribution of charged particles. A plate having through holes is provided to restrict the passage of the beam. The current distribution induced on the plate is then measured so that a uniform ion beam can be provided over a target area. The present invention, on the other hand, measures the current on an aperture plate so that the size of the aperture can be adjusted to correct for beam divergence.
U.S. Pat. No. 4,633,172 to Ekdahl, Jr. et al. shows an in-line beam current monitor for measuring total electron beam current. Unlike the present invention, Ekdahl, Jr. et al. are not concerned with measuring the current realized by electrically isolated aperture plates. The current measured by the present invention is indicative of grid condition and change in beam divergence. The present invention corrects the size of the aperture in accordance with the measured current.
U.S. Pat. No. 4,628,209 to Wittkower shows a device for measuring the spatial intensity distribution of a beam. Wittkower rotates a plate having targets separated by apertures. As the beam passes through the aperture, it is received by a Faraday cup. The current observed in the Faraday cup measures the beam intensity passing through each of the apertures. Unlike the present invention, Wittkower does not measure the current at the aperture plate for the purpose of adjusting the aperture size.
U.S. Pat. No. 4,135,097 to Forneris et al. and U.S. Pat. No. 4,118,630 to McKenna et al. each show an ion beam apparatus which controls the surface potential of a target surface. Forneris et al. and McKenna et al. measure the ion beam current of the target surface and adjacent electrically insulated walls in order to minimize the positive charge buildup on the insulated surface. The present invention is not concerned with the current at the target, but the current at the aperture plate.
U.S. Re. Pat. No. 33,193 to Yamaguchi et al. discloses an ion beam apparatus having an adjustable aperture. However, Yamaguchi et al. do not adjust the aperture in response to a measured current on the aperture.
The prior art references discussed above show conventional techniques for measuring beam current. However, these prior art references, do not measure the current from an aperture plate. The present invention, on the other hand, measures current from an isolated aperture plate for the purpose of adjusting the aperture size to correct for beam divergence. None of the references above seek to solve beam divergence. The present invention further utilizes the aperture current to signify grid condition.
Referring now to FIG. 1, an ion beam sputtering apparatus of the prior art is shown. In ion beam sputtering, ions are accelerated from a region in which they are generated, called the plasma source 10, by means of suitably electrified grids 12, 14, and directed at high energies onto a target 33. An ion beam 16 is generally indicated by an arrow. On impact, material from the target is sputtered off and subsequently received by suitably located substrates, resulting in the deposition of target material. An aperture plate 20 is generally employed in such a system. The aperture plate 20 is typically affixed to a mounting plate 22 by means of mounting posts 24 or other suitable mounting apparatus.
The aperture plate 20 serves at least two purposes. First, it intercepts parts of the ion beam 16 which are divergent from the main beam direction. These divergent parts may otherwise deleteriously impinge on materials other than the desired target materials inside the deposition chamber. Sputtering of such non-target materials in the deposition chamber is highly undesirable. Second, when depositing highly insulative materials, such as silicon dioxide, some amount of the target material may be deposited on the front 32, or target side, of the grid 12. Thus, transforming the surface of this grid from a conductor to an insulating coating. Such a coating may accumulate electrical charges to an extent that arcing occurs, with detrimental effects to the desired deposition process.
By intercepting a suitable amount of the ion beam, material 30 from the aperture plate 20 can be back sputtered onto the grid surface to an extent sufficient to maintain adequate conductivity on the grid surface. Such back sputtering permits the accumulating charges to bleed off harmlessly, thus preventing arcing at the grids 12, 14.
It is desirable to keep the size of the aperture in aperture plate 20 as large as possible while maintaining its functions. If the divergence of the ion beam changes, as, for example, from grid misalignments or from grid dimensional changes due to erosion during operation, the optimal aperture opening changes. Using the aperture of the prior art, it is difficult to monitor such changes either dynamically during a deposition process, or even statically between such processes.